Static on the Brain
Sometimes it’s easier to hear a quiet sound if there’s additional background static, or see a dim light if there’s another one flickering nearby. Now, in the 30 May issue of PRL, a team shows that this better perception translates into improved behavioral responses as well. Volunteers watching a barely visible but slowly varying light were also shown a flickering light. The noisy light boosted their perception, and the subjects also improved their ability to track the changing dim light with hand motions. The work could lead to “noisy goggles” that enhance vision with flickering lights.
Since the mid-1990s, researchers have found many situations where adding random fluctuations to a faint signal enhances perception–an effect known as stochastic resonance. One explanation, proposed by Yoshiharu Yamamoto of the University of Tokyo and PRESTO, the Japan Science and Technology Corporation, and his colleagues, is that the extra noise affects the brain’s electrically oscillating neurons. A dim light, for example, might get only a few of those neurons in the brain oscillating, but the noise somehow enhances or synchronizes more of those oscillations. More neural activity leads to greater perception.
The brain is a nonlinear system because output signals are not directly proportional to the inputs. Other nonlinear systems also appear to be enhanced by stochastic resonance. For example, noisy fluctuations may have amplified the effect of weak periodic wobbling in the Earth’s orbit and caused ice ages. “If we can show that noise can play a constructive role in nonlinear systems,” says Keiichi Kitajo of the University of Tokyo and the University of British Columbia in Canada, “it will help us understand how many nonlinear systems, including the human brain, work in noisy natural environments.”
Within the human body, most studies have tested only autonomous responses, showing, for example, that human neurological noise can help trigger changes in heart rate in response to small changes in blood pressure. Kitajo, Yamamoto, and their colleagues, set out to show that stochastic resonance could also affect larger behavioral responses. For each of their 19 subjects, the researchers presented the right eye with a light just barely too dim to be seen. The subject also had a hand control which he or she squeezed in relation to changes in the brightness of the dim light. Next, the team introduced the random noise–something like a randomly flickering halogen lamp. Every subject went through one trial where the noise and dim light went to the same eye and a second where the two signals went to opposite eyes.
Whenever the flickering light was on, the participants’ perception of the dim light improved, as shown by the increased correlation between the force of their squeeze on the handgrip and the dim light’s variations. The subjects’ perception improved regardless of which eye the noisy light came from, so the team concluded that the effect occurs in the brain, not in the eye. Previous studies have not been able to establish its location.
“Earlier work had been doing direct recording of the neurons to check only if the response of the neurons was improved,” says Jim Collins of Boston University. “This paper went beyond that to see if the subjects themselves are processing the information, to see if the subjects could demonstrate a better response.” Given that response, the team thinks goggles could be developed that would improve vision by adding fluctuating lights to the wearer’s view.
Karen Fox is a freelance science writer in Washington, DC, and the author of .